There are two kinds of typical rare-earth magnets, namely sintered magnets and quenched magnets produced by a melt spinning method.
A rare-earth sintered magnet having a Maximum Energy Product (MEP) of 216-296 kJ/m3 is widely used in a relatively large motor of which mechanical output is between a few hundreds of W and a few tens of kW. Such a large motor is used in a Magnetic Resonance Image (MRI), Voice Coil Motor (VCM), Factory Automation (FA), or Electric Vehicle (EV).
While, a small-diameter annular isotropic rare-earth bonded magnet is used in a permanent-magnet small motor. This bonded magnet has an MEP of 72 kJ/m3 or smaller, and is produced by fixing, through resin, RE-TM-B based quenched magnet powder that is obtained by crushing a melt span ribbon. A study for increasing the MEP of the isotropic rare-earth bonded magnet that is produced by crushing the melt span ribbon has not been significantly proceeding. Additionally, while increase in performance and added value of the electrical/electronic apparatus has been demanded, further decrease in size and weight and increase in output of the permanent-magnet motor have been always demanded.
For satisfying these demands, anisotropic bonded magnets have been actively developed. An anisotropic rare-earth bonded magnet having an MEP of 150 kJ/m3 is also produced. Anisotropic rare-earth magnet powder of which coercive force HCJ is 1.20 MA/m or higher—heat stability is expected—has also been developed. However, a rare-earth bonded magnet with a high MEP made of the anisotropic rare-earth magnet powder is a cylindrical or cubic prototype, and is hardly applied to an actual and general small motor. That is because a magnet to be mounted to a target small motor of the present invention is required to have not a simple cylindrical or cubic shape but an annular or circular arc small-diameter shape having a thickness of 1 mm or shorter. Further, for producing the annular magnet, a radial anisotropic rare-earth bonded magnet which has magnetic anisotropy in the radial direction is required. A generating method of a radially oriented magnetic field is disclosed in Japanese Patent Unexamined Publication No. S57-170501. This generating method employs a die where magnetic material yokes and non-magnetic material yokes are combined alternately around an annular die cavity and an exciting coil is disposed outside them. This method requires large magnetomotive force in order to generate the radially oriented magnetic field of a predetermined intensity in the annular die cavity. For effectively collecting magnetic fluxes, which are excited in the exciting coil by the magnetic material yokes, from the outer periphery to the inside of the annular die cavity, the magnetic path of the magnetic material yokes must be elongated. Especially, when the annular die cavity has a small diameter (or long size), a considerable percentage of the magnetomotive force is consumed as leakage fluxes. As a result, the oriented magnetic field of the annular die cavity decreases, and hence only an annular or circular arc rare-earth bonded magnet having a low MEP can be actually manufactured. This is different from the case where the prototyped cylindrical or cubic rare-earth bonded magnet has a high MEP.
Additionally, the compression molding pressure is high, namely 600-1000 MPa. Therefore, a new surface or micro-crack is apt to occur in anisotropic rare-earth magnet powder during molding, the rectangularity of a demagnetization curve can be reduced by permanent degradation by oxidation, and the magnetic characteristic can be reduced by increase in irreversible demagnetizing factor.